There are many methods known to the art to produce an article with a black appearance. These methods include black organic coatings, black phosphate conversion coatings, black oxide chemical treatments, black hexavalent and trivalent chromium passivation conversion coatings, black nickel deposition coatings, and heat (thermal) treatments. The heat treatments may be achieved by exposure to both gaseous atmospheres and molten salt treatments but have the disadvantage that their use is primarily tailored to obtain physical characteristics within the substrate material, however the corrosion resistance is often insufficient and applicability is limited to specific metal alloys.
Several types of phosphate conversion coatings are available, however, those based on zinc, manganese, or zinc-manganese or modifications of those produce a heavier film with a darker, more uniform appearance. These coatings are generally non-metallic and crystalline in form and have limited corrosion resistance without application of subsequent inhibitor, oil, wax, or polymeric coatings.
The use of black chromate conversion coatings, particularly of the hexavalent type, has been discouraged and in some places banned altogether. A further disadvantage of hexavalent chromate films is the significant decline in corrosion resistance occurring when the films are exposed to temperatures above approximately 75 C. The performance of trivalent black chromate conversion coatings has thus far been inconsistent and too expensive for some applications. Black oxide treatments and chromium passivation treatments may additionally include the use of black dyes. Black nickel coatings are well known for decorative applications but heretofore have not typically been utilized for applications requiring corrosion resistance.
Organic coatings may be applied via spray, immersion (dip-drain and dip-spin), and brush application. Immersion coatings include E-Coat applications (electrolytic deposition) and autodeposition applications (also known as autophoretic, i.e. catalytically deposited coatings). Spray application includes powder coating which is an electrostatic deposition as well as liquid paint propelled with compressed air or other propellants. The organic coatings are cured by use of numerous techniques which include coalescence upon evaporation of water or other solvent, thermal curing, reactive curing (moisture, UV/Radiation, chemical cross-linkers, etc.). The most significant drawback of the aforementioned commercially available black surface treatments is that, with exception to the chromate treatments, they provide either insufficient corrosion resistance, excessively thick polymer layers, or their appearance is non-uniform due to article to article contact during their treatment process. The non-uniform appearance can be overcome with successive coating layers such as in the case of organic coatings, however, the resulting excessive thickness cannot be tolerated on fasteners with fine threads or other articles with critical dimensions.
Various treatments, which incorporate silicon, are also well known in industry. These treatments may be of the form where silicon is contained in an organic molecule such as of the silane type (e.g. aminosilanes and epoxy silanes) or organosilicate type (e.g. tetraethyl ortho silicate), organic polymers (e.g. silicone resins), and inorganic silicates and silicas. However, with exception to the silicone resins which generally result in a coating that is too thick for fasteners, the films and surface treatments consisting of various silicon containing materials does not effectively produce a uniform black coating. Applicability of electrolytic deposition of silicate thin films has been limited to electrically conducive metallic substrates. There is a need in this art for a black non-chromate that is obtained from environmentally acceptable materials.